Stacked color liquid crystal display device

ABSTRACT

A liquid crystal display device includes cell wall structure and a chiral nematic liquid crystal material. The cell wall structure and the liquid crystal cooperate to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field to the liquid crystal for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures. The liquid crystal material has a pitch length effective to reflect radiation having a wavelength in both the visible and the infrared ranges of the electromagnetic spectrum at intensity that is sufficient for viewing by an observer. One liquid crystal material may be disposed in a single region or two or more liquid crystal materials may be used, each in separate regions even without the infrared reflecting layer. One aspect of the invention is directed to a photolithography method for patterning a substrate of the display. The display may also have multicolor capabilities by including separate layers of at least two or three liquid crystal materials that reflect visible light. A full color stacked display may be produced with grey scale capabilities.

This application is a divisional of U.S. patent application Ser. No.09/330,104 filed on Jun. 10, 1999, now U.S. Pat. No. 6,654,080 which isa continuation of application of U.S. patent application Ser. No.08/823,329, filed on Mar. 22, 1997, now U.S. Pat. No. 6,034,752, issuedon Mar. 7, 2000.

This application was made in part with United States Government supportunder cooperative agreement N61331-96C-0042 awarded by the DefenseAdvanced Research Projects Agency (DARPA). The government has certainrights in this invention.

BACKGROUND OF THE INVENTION Field of the Invention

Cholesteric displays are bistable in the absence of a field, the twostable textures being the reflective planar texture and the weaklyscattering focal conic texture. In the planar texture, the helical axesof the cholesteric liquid crystal molecules are substantially parallelto the substrates between which the liquid crystal is disposed. In thefocal conic state the helical axes of the liquid crystal molecules aregenerally randomly oriented. By adjusting the concentration of chiraldopants in the cholesteric material, the pitch length of the moleculesand thus, the wavelength of radiation that they will reflect, can beadjusted. Cholesteric materials that reflect infrared radiation havebeen used for purposes of scientific study. Commercial displays arefabricated from cholesteric materials that reflect visible light.

Liquid crystal displays are useful as instrumentation in vehicles. Forexample, commercial airlines employ LCD instrumentation in the cockpits.Vehicles such as for military use, may use LED or LCD instrumentation.In military vehicles used to conduct stealth night operations, such asarmy helicopters, pilots wear night vision detectors or goggles thatenable them to view objects in the air and on the ground without usingvisible light. The night vision goggles enable the wearer to viewinfrared radiation, such as the heat from the motor of an automobile.The night vision goggles may also utilize the ambient infrared lightfrom the night sky to view objects that do not emit infrared radiation.The night vision goggles are worn spaced from the eyes of the pilot sothat the LED instrumentation panels can be read when the wearer looksdown, without looking through the goggles. Use of current night visiongoggles limits the depth perception of the wearer. In addition, visiblelight may saturate the night vision goggles and render them ineffective.The goggles thus may filter out certain wavelengths of visible light.

SUMMARY OF THE INVENTION

The present invention is directed to a liquid crystal display includinga single chiral nematic liquid crystal material or at least two chiralnematic liquid crystal materials, which can reflect light across aparticular range of wavelengths. One aspect of the invention is directedto a display that reflects in both the visible and infrared ranges ofthe electromagnetic spectrum at an intensity that is observable to thehuman eye. The radiation in the infrared spectrum is observed using adevice suitable for detecting infrared radiation, such as night visiongoggles. Another aspect of the invention is the cell wall configurationsof the displays. The invention may employ a single cell using one chiralnematic liquid crystal that reflects in the visible and in the infraredranges or at least two liquid crystal cells each having a differentchiral nematic liquid crystal disposed in each. When two or more cellsare used, the cells may be stacked on top of one another. The chiralnematic liquid crystal composition may be tailored to have a certainpeak intensity and bandwidth according to the invention. Although thepreferred operation of the inventive display utilizes light reflectingfrom the liquid crystal, it would be appreciated by those skilled in theart in view of this disclosure that the display may be used in atransmissive mode using backlighting.

In general, the present invention is directed to a liquid crystaldisplay device comprising cell wall structure and a chiral nematicliquid crystal material. The cell wall structure and the liquid crystalcooperate to form focal conic and twisted planar textures that arestable in the absence of a field. A device applies an electric field tothe liquid crystal for transforming at least a portion of the materialto at least one of the focal conic and twisted planar textures. Theliquid crystal material reflects radiation having a wavelength in boththe visible and the infrared ranges of the electromagnetic spectrum atintensity that is sufficient for viewing by an observer. In particular,the liquid crystal has a positive dielectric anisotropy. At least about20% of the radiation incident on the material is preferably reflectedfrom the material. The liquid crystal material may have an opticalanisotropy of at least about 0.10.

In one embodiment of the invention, the display device employs a singleliquid crystal material that is disposed in one region and yet reflectsboth visible and infrared radiation. This is accomplished by selectingthe peak reflection wavelength of the radiation and by broadening thebandwidth or the range of wavelengths in which the radiation isreflected.

Another embodiment of the invention utilizes two regions, a liquidcrystal material being disposed in each region. The cell wall structureforms a first region in which a first chiral nematic liquid crystalmaterial is disposed and a second region in which a second chiralnematic liquid crystal material is disposed. The first liquid crystalreflects radiation having a wavelength in the visible range and thesecond liquid crystal material reflects radiation having a wavelength inthe infrared range.

The particular cell wall structure that is used to form the two regionsmay be a stacked display employing three, four or more substrates. Theliquid crystal material is disposed between opposing substrates. In oneaspect using four substrates the first region is disposed between firstand second substrates and the second region is disposed between thirdand fourth substrates. The first and second regions are arranged inseries with respect to one another in the direction toward the observer.In this regard, the first liquid crystal reflecting visible light isdisposed downstream of the liquid crystal reflecting infrared radiationin the direction toward the observer. In the case of the three substratestacked display, the first region is disposed between first and secondsubstrates and the second region is disposed between the secondsubstrate and a third substrate.

The spacing between substrates in the single cell display ranges fromabout 4 to about 10 microns. The spacing between the substrates in thestacked cell display is at least about 4 microns.

The three cell display employs a photolithography method of the presentinvention to form a substrate that employs patterned electrodes on bothsides. This substrate can be used in any stacked display. This method ofthe invention includes applying radiation in the ultraviolet region ofthe electromagnetic spectrum through a mask. The radiation is reflectedthrough a substrate, each opposing surface of the substrate containing alayer of photoresist material over a conductive layer disposed on thesurface. The photoresist layer is exposed to the UV radiation on bothsides of the substrate. Exposed photoresist material and underlyingelectrode material are removed from the substrate to form an electrodepattern on both surfaces of the substrate.

In particular, the ultraviolet radiation is applied at a level effectiveto compensate for the transmission loss of the photoresist, theelectrode and the substrate.

The ultraviolet radiation is applied at a level that is at least twotimes the level of ultraviolet radiation that is normally used to exposephotoresist on a substrate.

A preferred embodiment of the present invention is directed toinstrumentation of the type that is used by personnel employing a nightvision detector such as goggles. The instrumentation reflects lighthaving a wavelength in the visible region of the electromagneticspectrum. This embodiment of the present invention has militaryapplications, such as use in instruments in the cockpit of armyhelicopters. The present invention includes a liquid crystal displaydevice comprising cell wall structure and a chiral nematic liquidcrystal material. The cell wall structure and the liquid crystalcooperate to form focal conic and twisted planar textures that arestable in the absence of a field. A device applies an electric field tothe liquid crystal for transforming at least a portion of the materialto at least one of the focal conic and twisted planar textures. Theliquid crystal material can reflect radiation having a wavelength in thevisible and infrared regions of the spectrum at an intensity sufficientfor viewing by the personnel.

Another aspect of the present invention is a multicolor stacked celldisplay that reflects infrared and visible radiation. The display devicecomprises cell wall structure and a chiral nematic liquid crystalmaterial. The cell wall structure and the liquid crystal cooperate toform focal conic and twisted planar textures that are stable in theabsence of a field. The cell wall structure forms first, second andthird regions in which first, second and third chiral nematic liquidcrystal materials are disposed, respectively. A device applies anelectric field to at least one of the first, second and third liquidcrystal materials for transforming at least a portion of these materialsto at least one of the focal conic and twisted planar textures. Thefirst and second liquid crystal materials have a pitch length effectiveto reflect radiation in the visible range of the electromagneticspectrum and the third liquid crystal has a pitch length effective toreflect radiation in the infrared range of the spectrum. The visible andinfrared radiation has an intensity sufficient for viewing by anobserver.

Particular features of the color display are that the first liquidcrystal may have a pitch length effective to reflect light of a firstcolor and the second liquid crystal material may have a pitch lengtheffective to reflect light of a second color. The display may include atleast one other region in which a liquid crystal material that canreflect light in the visible range. For example, three visible cells maybe used, resulting in a full color display.

In the stacked color display, when using substrates having patternedelectrodes on only one side, the first region is disposed between firstand second substrates, the second region is disposed between third andfourth substrates and the third region is disposed between fifth andsixth substrates. Alternatively, when using a substrate with electrodespatterned on both sides, the first region is disposed between first andsecond substrates, the second region is disposed between the secondsubstrate and a third substrate and the third region is disposed betweenthe third substrate and a fourth substrate. The first and the secondregions are disposed downstream of the third region with respect to thedirection from the display toward the observer. The invention may alsoinclude at least one colored material layer or a black layer transparentto infrared radiation. The colored material is disposed at the backsubstrate of a visible cell that is adjacent the infrared cell. Theinfrared transparent black layer may be disposed at the back of avisible cell. Also, a black layer may be adjacent the rearmost substrateof the infrared cell.

A method of making a display that can reflect infrared and visibleradiation according to the invention includes adjusting the pitch lengthof a chiral nematic liquid crystal material so that the materialreflects radiation having a wavelength in the visible and in theinfrared ranges of the electromagnetic spectrum. Opposing substrates arespaced apart at a distance effective to provide the visible and infraredradiation with an intensity sufficient for viewing by an observer. Thematerial is filled between the substrates such that the cell wallstructure cooperates with the liquid crystal to form focal conic andtwisted planar textures that are stable in the absence of a field. Also,connected is device for applying an electric field to the liquid crystalfor transforming at least a portion of the material to at least one ofthe focal conic and twisted planar textures. The bandwidth ofreflectance from the display may be broadened by using liquid crystalmaterial having an optical anisotropy of at least about 0.10.

The present invention offers numerous features and advantages that haveheretofore not been possible. A display reflecting both visible andinfrared radiation enables use during the night and day, withoutcompromising the electrooptical characteristics of the display.Moreover, the stacked cell feature of the invention enables ease ofmanufacture and modification for various applications. For example,variations in color and contrast may be attained utilizing colored orblack layers on one or more of the substrates. Both cells of any stackeddisplay, by tailoring the chiral nematic liquid crystal material in eachcell, may be operated utilizing the same waveforms and driving voltages.

The photolithography method of the invention reduces the scattering ofthe stacked display. Also, no index matching material is needed betweensubstrates. The method exposes the photoresist on both sides of thesubstrate using a single exposure step. Without this step, separatephotolithography and wet chemical etching would have to be performed oneach side of the substrate to pattern the electrodes. Also, for highresolution displays greater than 100 dots per inch, the electrodepatterns must be registered to within 10 microns to avoid parallaxproblems. The double exposure technique cuts the photolithography andetching steps in half while automatically aligning the electrodepatterns, since only one UV exposure is used to expose photoresistcoated on both sides of the substrate.

The display of the invention may employ frontlighting and can utilizeambient visible or infrared radiation. Those skilled in the art wouldalso appreciate that the invention may be modified to be suitable forbacklighting. The display may be fabricated to include a device fordirecting either visible or infrared radiation onto the display.Alternatively, infrared radiation may be reflected from the night visiongoggles toward the infrared reflecting display. In the case of militaryvehicles such as helicopters, the apparatuses surrounding the cockpit,for example, may provide ambient infrared radiation sufficient toilluminate the display. So, too, may visible light from instrumentationin the cockpit be sufficient to illuminate the visible display.

The present invention would be useful in any application in which it isdesirable to have a display reflecting in the infrared and visibleranges. The invention may be suitable for use in instrumentation inhelicopter or airplane cockpits, such as those that include numericaldisplays. Other applications include a display that can reflect infraredand visible light for use in a global positioning system that enablesthe user to determine his location based upon satellite information.Such a display could be used by foot soldiers employing night visiongoggles who can read the display using only infrared radiation. Ininstances in which night vision goggles are used, since the wearer canview the infrared reflecting display through the goggles, the gogglesmay be worn closer to the face. This may improve viewing through thegoggles.

Many additional features, advantages and a fuller understanding of theinvention will be had from the accompanying drawings and the detaileddescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the reflectance as a function of wavelengthfor a cell that reflects visible light and a cell that reflects infraredlight, constructed according to the present invention;

FIG. 2 shows the spectral sensitivity of infrared detecting goggles;

FIG. 3 shows an electrooptical response of a cell that reflects visiblelight;

FIG. 4 shows the relaxation time of a cell that reflects visible light;

FIG. 5 shows a stacked display employing four substrates and a cell thatreflects visible light and a cell that reflects infrared radiation,constructed according to the present invention;

FIG. 6 shows a stacked display employing three substrates and a cellthat reflects visible light and a cell that reflects infrared radiation,constructed according to the present invention;

FIG. 7 shows a photolithography method of making a substrate havingpatterned electrodes on both sides, according to the present invention;

FIG. 8 shows the transmission of a photoresistive material;

FIG. 9 shows the transmission of a glass substrate with electrodecoatings on both sides;

FIG. 10 shows electronics for the display shown in FIG. 6;

FIG. 11 shows a stacked display having multicolor capabilitiesconstructed according to the present invention, including three cellsthat reflect visible light and a cell that reflects infrared radiation;and

FIG. 12 is a plot of the electro-optic response of a cell to AC pulsesof varying voltages, which demonstrates grey scale characteristics.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention is directed to a liquid crystal display devicethat comprises cell wall structure and chiral nematic liquid crystalmaterial having positive dielectric anisotropy. The cell wall structureand the liquid crystal cooperate to form focal conic and twisted planartextures that are stable in the absence of a field. A device enables anelectric field to be applied to the liquid crystal for transforming atleast a portion of the material to at least one of the focal conic andtwisted planar textures. Upon applying the electric field the liquidcrystal material can reflect radiation having a wavelength in both thevisible and infrared ranges of the electromagnetic spectrum at intensitysufficient for viewing by an observer.

The concentration of the chiral material is selected to provide thecomposition with a pitch length effective to reflect radiation of apredetermined wavelength. The concentration of chiral material isselected so that the display reflects radiation in the infrared region.Reference to the infrared region herein is the region of the spectrumhaving a wavelength of at least about 700 nanometers (nm) and, inparticular, at least about 780 nm. The concentration of chiral materialis also selected so that the display reflects radiation in the visibleregion. Reference to the visible region herein means the region of thespectrum having a wavelength that is not greater than 780 nm and, morepreferably, the wavelength region ranging from about 400 to about 650nm. The chiral material can also be present in an amount that produces apitch length effective to reflect visible light of desired colors.

The displays of the invention may employ different cell wallconfigurations. The invention may employ a single cell using only onechiral nematic liquid crystal material between opposing substratesreflecting both visible and infrared radiation. The display may alsoinclude at least two liquid crystal cells each including a differentchiral nematic liquid crystal. When using at least two cells, one cellincludes a chiral nematic liquid crystal material having a pitch lengtheffective to enable the liquid crystal to reflect visible light and theother cell includes a chiral nematic liquid crystal having a pitchlength effective to enable the liquid crystal to reflect infraredradiation.

The pitch length of the liquid crystal materials of the invention areadjusted based upon the following equation (1):λ_(max) =n _(av) ·p ₀  (1)

where λ_(max) is the peak reflection wavelength (wavelength at whichreflectance is a maximum), n_(av) is the average index of refraction ofthe liquid crystal material, and p₀ is the natural pitch length of thecholesteric helix.

Definitions of cholesteric helix and pitch length and methods of itsmeasurement, are known to those skilled in the art such as can be foundin the book, Blinov, L. M., Electro-optical and Magneto-OpticalProperties of Liquid Crystals, John Wiley & Sons Ltd.

1983. The pitch length is modified by adjusting the concentration of thechiral material in the liquid crystal composition. For mostconcentrations of chiral dopants, the pitch length induced by the dopantis inversely proportional to the concentration of the dopant. Theproportionality constant is given by the following equation (2):P ₀=1/(HTP.c)  (2)

where c is the concentration in % by weight of the chiral dopant and HTPis the proportionality constant.

When filled into a single cell, the pitch length is adjusted to enablethe device to reflect radiation in both the visible and the infraredregions of a sufficient intensity that can be observed by the human eye.For the cell to reflect in the infrared spectrum, λ_(max) is preferablyin the infrared region and is preferably within Δλ/2 of the infraredregion, where Δλ is the bandwidth of the reflection peak. This ensuresthat enough light is reflected to achieve suitable contrast. In thisregard, it is preferable to design the chiral nematic composition of thesingle cell display so that the device reflects radiation of about 700nm. Those skilled in the art would appreciate in view of this disclosurethat the maximum wavelength peak may have a wavelength that is loweredfurther into the visible region if reflecting a broader range or ahigher intensity of visible light is desired. Conversely, the maximumwavelength peak may have a wavelength that is increased further into theinfrared range if reflecting a broader range or a higher intensity ofinfrared radiation is desired.

A chiral nematic liquid crystal has a relatively long pitch length inorder to reflect infrared radiation compared to a composition thatreflects visible light. In a two cell display, the pitch length of thecomposition reflecting infrared radiation is adjusted to be longer thanthe pitch length of the composition reflecting visible light. Thespacing between opposing cell walls must be widened to accommodate alonger pitch length so that a desired number of pitches is obtained inthe cell. The number of pitches generally believed desirable for a cellto have sufficient reflectance or brightness and thus, contrast, isabout 12–15. In a single cell, the cell spacing must be adjusted so thatthe reflected radiation has an intensity that is high enough to beobserved. This is also true in the case of the infrared cell of thestacked display—a longer cell spacing is used.

The reflection spectrum of a cholesteric material typically has a fullwidth at half maximum (FWHM) on the order of about 100 nm for a pitchlength that enables the liquid crystal to reflect yellow light. Thebandwidth may be widened as desired in the case of the single cell, sothat part of the reflection curve is in the visible region and part ofthe reflection curve is in the infrared region.

In a single cell display, the typical 100 nm bandwidth is not wideenough to achieve good contrast with respect to both the visible and theinfrared ranges. Therefore, the liquid crystal composition is tailoredto broaden the reflection bandwidth. The width of the reflection band ofthe chiral nematic liquid crystal is given by the following equation(3):Δλ=p₀ ·Δn  (3)

where Δn is the optical anisotropy of the liquid crystal and Δλ is thebandwidth.

Increasing the pitch length will broaden Δλ. However, Δλ may also bebroadened by increasing the Δn of the liquid crystal. To enable thesingle cell display to be suitable for reflecting radiation in both thevisible and infrared regions, the chiral nematic composition is tailoredto have an optical anisotropy of at least about 0.10. It should bewithin the purview of the skilled chemist to prepare a chiral nematicliquid crystal composition having an optical anisotropy of at leastabout 0.10. The operation of a visible display and an infrared displayis shown in FIG. 1. It will be understood that all of the graphs of thisapplication show aspects of the performance of the displays of thepresent invention for purposes of explanation only and are not intendedto show the optimum or ideal performance of the displays of the presentinvention or the conditions of their use. The composition of the visibleand infrared cells of FIG. 1 are the same as provided in Example 1hereafter. Although the data of FIG. 1 was derived from separate visibleand infrared cells that were not stacked, it illustrates the expectedbehavior for both the single cell display and the stacked display. Anintegrating sphere was used to measure the spectral reflectance underdiffuse illumination.

In FIG. 1 the oscillations of the focal conic (weakly scattering)textures are caused by the interference of the light due to thesubstrates. Curve A is the reflectance from a cell designed to reflectvisible light. Curve B is the reflectance from a cell designed toreflect infrared radiation. The cells were in the reflective twistedplanar state when the curves A and B were produced. The measurementswere conducted with black paint on the back of the visible cell and theinfrared cell to improve contrast.

The peak reflectance of both displays is surprisingly at least about 20%reflectance and, in particular, between at least about 25% and 30%reflectance. It is unexpected that such a high reflectance can beobtained from a cell reflecting infrared radiation. This relatively highreflectance of the infrared cell was obtained through the use of arelatively large cell spacing. FIG. 1 illustrates that the spectralbandwidth changes as the pitch length, and hence the peak wavelength ofreflected radiation, changes. The curve B had a greater bandwidth FWHM(170 nm) than the curve A bandwidth FWHM (100 nm).

One advantageous feature of the stacked display is the effect that eachof the planar infrared and visible curves have in the other's region ofthe spectrum. The portion C of the planar infrared curve has areflectance at wavelengths below 650 nm of greater than 5%. The portionof the planar visible curve D has a reflectance at wavelengths above 700nm that is less than 5%. The infrared cell scatters light in the visibleregion, but the visible cell does not scatter much in the infraredregion. Positioning the visible cell in front of the infrared cell thusprovides it with better contrast. However, the infrared cell may belocated in front of the visible cell despite these concerns.

The rearmost substrate of each display is preferably painted black. Theblack paint absorbs infrared radiation that reaches the back of thedisplay. In the case of the stacked cell display, the contrast may beimproved by painting the back substrate of the last visible cell black.The paint should be transparent to infrared radiation. This effectivelyprovides the visible cell with a black background that improves itscontrast, and yet, does not alter the viewing characteristics of theinfrared display. Paint such as black paint, which is transparent in theinfrared region, is known to those skilled in the art. For example, manytypes of black paint used to print the letters on computer keys aretransparent to infrared radiation. The substrates of a visible cell mayalso be painted other colors. The substrates are comprised of glass orplastic as is known to those skilled in the art. Glass substrates maycomprise fused silica, soda lime glass or borosilicate glass, forexample.

Infrared detectors such as night vision goggles typically employ filtersthat remove unwanted visible light. In addition to carefully tailoringthe liquid crystal composition of the single cell display to obtain asuitably broad bandwidth, the infrared detector may need to be tuned foruse with the single cell display of the present invention, to adjust theabsorption wavelengths of its visible light filters and allow selectedvisible light to pass through. For example, the goggles may be tuned toallow visible light having red and yellow wavelengths to pass through.

When the goggles are used with the stacked cell display, the display maybe tailored to meet the manufacturer's specifications regarding thespectral sensitivity of the detector. FIG. 2 illustrates a typicalspectral sensitivity of Class A night vision goggles, model numberAN/PVS-7B, manufacturer's identification No. 66868-300030-1,manufacturer unknown. This Figure shows that although there is not aperfect correlation between the current sensitivity of the goggles andthe reflectance curve of the infrared display, these goggles would workwell with a single cell display having a peak wavelength λ_(max) ofabout 700 nm. If the stacked cell display of the invention is intendedto be used with the goggles of FIG. 2, the infrared curve may be movedso that its peak λ_(max) is centered over the region of the goggles,ie., at about 750 nm.

FIG. 3 shows the electrooptical characteristics of a visible cell. Thiscurve was prepared using a visible cell having a liquid crystalcomposition provided in Example 1 hereafter. Curve E shows the cell inthe planar reflecting state before the pulse whereas curve F shows thecell in the scattering focal conic state before the pulse. Theelectrooptical response curve of the infrared cell is very similar tothat of the visible cell and both curves have the same voltage levels.This graph was obtained in a conventional manner using a monochromelight source, the voltage being applied in two volt steps. The voltageknown as V₃ needed to drive the material shown in the curve E from thebright planar state to the dark, mostly focal conic state was about 32volts AC. The voltage known as V₄ needed to drive the material of thecurve F from the dark focal conic state to the bright reflective state,was about 42 volts AC.

Since the electrooptical response characteristics of the infrared andvisible cells of a stacked display have been matched with similardriving voltage levels, the two displays may be driven simultaneously,independent of the method of coupling the two cells. The same waveformsand controllers can be used to drive both the visible and infrared cellsof the stacked display. The viscosity of the cell is influenced by thepitch length. One would not expect that the two cells could use similardriving voltages. The viscosity of the infrared cell is less than thatof the visible cell due to its longer pitch length. This lower viscosityrequires a lesser driving voltage. However, using a larger spacing ofthe substrates of the infrared cell requires a greater driving voltage.As a result of the lowered viscosity but greater cell spacing of theinfrared cell, the driving voltage of the visible and infrared cells aresimilar.

FIG. 4 shows a graph of reflectance as a function of time (ms) for avisible cell and illustrates the relaxation time from the homeotropictexture to the planar texture. This curve was prepared using a visiblecell having a liquid crystal composition provided in Example 1hereafter. FIG. 4 was obtained by using an AC voltage having a 100 mspulse width. The pulse was applied at about 580 ms and turned off atabout 680 ms. The relaxation time will be shorter for the infrared cell.

The present invention may employ any suitable driving schemes andelectronics known to those skilled in the art, including but not limitedto the following, all of which are incorporated herein by reference intheir entireties: Doane, J. W., Yang, D. K., Front-lit Flat PanelDisplay from Polymer Stabilized Cholesteric Textures, Japan Display 92,Hiroshima October 1992; Yang, D. K. and Doane, J. W., Cholesteric LiquidCrystal/Polymer Gel Dispersion: Reflective Display Application, SIDTechnical Paper Digest, Vol XXIII, May 1992, p. 759, et seq.; U.S.patent application Ser. No. 08/390,068, filed Feb. 17, 1995, entitled“Dynamic Drive Method and Apparatus for a Bistable Liquid CrystalDisplay” and U.S. Pat. No. 5,453,863, entitled “Multistable ChiralNematic Displays.” A passive matrix multiplexing type display ispreferably used in the present invention. The effect that pulseamplitudes and widths, and speeds of field removal have on each textureis described in the U.S. Pat. No. 5,453,863.

The liquid crystal of the present invention is addressed by applying anelectric field having a preferably square wave pulse of a desired width.The voltage that is used is preferably an AC voltage having a frequencythat may range from about 125 Hz to about 2 kHz. Various pulse widthsmay be used, such as a pulse width ranging from about 6 ms to about 50ms. The present invention may utilize the addressing techniquesdescribed in the U.S. Pat. No. 5,453,863 to effect grey scale.

The display of the invention may utilize ambient visible and infraredradiation or an illumination source on the display or on the nightvision goggles. The radiation incident upon typical cholesteric displayshas components that correspond to the peak wavelength of the display.One way to illuminate a cell to reflect infrared radiation is to shineinfrared radiation upon the display. In military applications such asfor use on instrumentation in the cockpit of a military helicopter, theilluminating radiation may be infrared only, which preserves thedarkness of the cockpit. It may also be possible to utilize the infraredcontent of the night sky derived in part from the moon and the stars.The infrared radiation of the night sky may even be sufficient on anovercast night because the infrared radiation may filter through theclouds.

An example of a single cell display is shown in U.S. Pat. No. 5,453,863,entitled Multistable Chiral Nematic Displays, which is incorporatedherein by reference in its entirety. The spacing between the substratesof the single cell display may range from about 4 microns to about 10microns.

One example of a display having two stacked cells is shown generally at10 in FIG. 5. This particular display employs four glass substrates 12,14, 16 and 18.

One cell 20 includes a first chiral nematic liquid crystal material 22disposed between the opposing substrates 12 and 14. The substrate 12 isnearest an observer. Another cell 24 on which the cell 20 is stackedincludes a second chiral nematic liquid crystal 26 disposed between theopposing substrates 16 and 18.

The first liquid crystal 22 includes a concentration of chiral materialthat provides a pitch length effective to enable the material to reflectvisible light. The second liquid crystal 26 includes a concentration ofchiral material that provides the material with a pitch length effectiveto enable the material to reflect infrared radiation.

The substrates 12, 14, 16 and 18 each have a patterned electrode such asindium tin oxide (ITO), a passivation material and an alignment layer28, 30, 32, respectively. The back or outside of the substrate 18 iscoated with black paint 34. The purpose of the ITO electrode,passivation material and alignment layer will be explained hereafter.

An index of refraction-matching material 36 is disposed between thesubstrates 14 and 16. This material may be an adhesive, a pressuresensitive material, a thermoplastic material or an index matching fluid.The adhesive may be Norland 65 by Norland Optical Adhesives. Thethermoplastic material may be a thermoplastic adhesive such as anadhesive known as Meltmount, by R. P. Cargile Laboratories, Inc. Thisthermoplastic adhesive may have an index of refraction of about 1.66.The index matching fluid may be glycerol, for example. When an indexmatching fluid is used, an independent method of adhering the two cellstogether is employed. Since both textures of the second cell aretransparent to visible light, the stacking of the cells does not requireaccurate alignment or registration of the two cells. The spacing betweenthe substrates 12 and 14 of the first cell ranges from about 4 to about6 microns. The spacing between the substrates 16 and 18 of the secondcell ranges from about 4 to about 10 microns and greater.

The driver circuitry 45 is electrically coupled to four electrode arraysE1, E2, E3 and E4, which allow the textures of regions of the liquidcrystal display to be individually controlled. As discussed in the priorart, application of a voltage across the liquid crystal material is usedto adjust the texture of a picture element. The electrode matrix E1 ismade up of multiple spaced apart conductive electrodes all orientedparallel to each other and all individually addressable by the driverelectronics 45. The electrode array E2 spaced on the opposite side ofthe liquid crystal material 22 has an electrode array of spaced apartparallel electrodes. These electrodes are arranged at right angles tothe electrodes of the matrix E1. In a similar manner the matrix array E3has elongated individual electrodes at right angles to the elongatedindividual electrodes of the matrix array E4.

Another stacked cell display is generally shown as 40 in FIG. 6. Thisdisplay 40 includes a visible cell 42 and an infrared cell 44 andincludes substrates 46, 48 and 50. A third chiral nematic liquid crystal52 is disposed between the substrates 46 and 48 of the visible cell. Thesubstrate 46 is nearest the observer. A fourth chiral nematic material54 is disposed between the substrates 48 and 50 of the infrared cell.

The third liquid crystal has a concentration of chiral additive thatprovides it with a pitch length effective to reflect visible light. Thefourth liquid crystal material has a pitch length effective to reflectinfrared radiation.

The spacing between the substrates 46 and 48 of the visible cell rangesfrom about 4 to about 6 microns. The spacing between the substrates 48and 50 of the infrared cell ranges from about 4 to about 10 microns andgreater.

The third and fourth liquid crystal materials may be the same ordifferent than the first and second liquid crystal materials. Thevisible cell 42 is preferably disposed downstream of the infrared cellin the direction from the infrared cell toward the observer. No indexmatching material needs to be used in the three substrate stackeddisplay.

The three substrate stacked display 40 is fabricated by a methodaccording to the present invention. In the three substrate display shownin FIG. 6, the middle substrate 48 is disposed between the substrates 46and 50 and is in common with the visible and infrared cells. The middlesubstrate 48 acts as the back substrate of the visible cell and thefront substrate of the infrared cell. The common substrate 48 hasconductive, passivation, and alignment layers 56, 58 and 60,respectively, coated on both sides. By passivation layer is meant aninsulating layer that prevents front to back shorting of the electrodes.The substrates 46 and 50 have patterned electrode, passivation, andalignment layers 56, 58 and 60 coated on only one side.

The fabrication of the three substrate display utilizes an inventivephotolithographic technique. Photoresist material, passivation materialand an alignment material are applied to the substrate by spin coating.The alignment material is used for providing the liquid crystalmolecules with a generally homeotropic texture adjacent the substratefor stabilizing the focal conic texture. The spin coating process isconducted at a rotational speed of several thousand rpm for about 30seconds, each time in this process. The soft bake is conducted at about90° C. each time in this process. To enable application by spin coating,the photoresist, passivation and alignment materials include solutes ofthese materials in solution.

The conductive coating is preferably comprised of transparent indium tinoxide (ITO), however, any conductive coating having good opticaltransmission may be utilized, such as conductive polymers and tin oxide.One example of a suitable passivation material is a SiO₂-like materialknown as NHC-720A, which is manufactured by Nissan Chemical. Thealignment material is manufactured by Nissan Chemical No. SE-7511L. Thephotoresist coating is spin coated to a thickness of about 2 microns ±½micron. The passivation layer is spin coated to a thickness ranging fromabout 400 to about 1000 angstroms, and, in particular, in this processat about 400 angstroms. The alignment layer is spin coated to athickness of about 250 angstroms ±50 angstroms.

Photoresist material 62 is first spin coated onto one side of thesubstrate 48 and then soft baked on a hotplate to flash off the solventsin the photoresist material. The substrate is then removed from thehotplate and cooled to room temperature. The substrate is then flippedover and photoresist material 62 is coated onto the opposite side. Thisside is then soft baked on a hotplate and cooled to room temperature.The substrate is then exposed to ultraviolet (UV) radiation R through amask 64 as shown in FIG. 7. One particular mask had thickness ofelongated strips of material of about 245 microns and a spacing betweenstrips of about 9 microns. The mask may comprise chrome oxide or ironoxide made by Hoescht. The UV radiation exposes the photoresist 62 onboth sides of the substrate 48, since the substrate, the conductivecoatings 56 and the layer of photoresist 62 transmit UV radiationthrough to the lower surface of the substrate.

The dose of UV radiation must be increased about four times above thelevel normally required to expose a single layer so that the bottomphotoresist layer is completely exposed. This is to correct for theoptical transmission loss of the substrate, ITO coatings and photoresistFIG. 8 shows the optical transmission of photoresist AZ 1518 obtainedfrom Hoescht, as a function of wavelength. The average opticaltransmission in the spectral region in the range of from 365 to 436 nmis about 40% after exposure to broadband UV radiation at a dose of 150milli-Joules/centimeter². FIG. 9 shows the optical transmission of thesecond substrate having 20 mil ITO coatings on both sides, as a functionof wavelength. The optical transmission was about 70% to radiationhaving wavelengths greater than about 400 nm. The optical transmissionof the ITO coated-substrate/photoresist combination is the product ofthe 40% optical transmission and the 70% optical transmission, or about28%. This illustrates that the dose must be increased by a factor ofabout four to insure 100% dose on the bottom substrate with the additionof a small margin, using these materials.

The value of the UV dose is that which is sufficient to compensate forthe optical transmission loss due to the electrode-coated glasssubstrate and photoresist. The UV dose required to expose the bottomphotoresist depends upon the composition of the ITO coating, the glasssubstrate and the photoresist, since not all commercially availableproducts will have the same optical transmission. The following Table 1illustrates the transmission of a fused silica substrate and a hightransmission ITO material, which may be obtained from the company,General Vacuum.

Trans- mission (%) Trans- Trans- of mission (%) mission (%) Trans-substrate Wavelength of bare of ITO + mission (%) with ITO on (nm)substrate substrate of ITO both sides 365 92 85 92 78 400 92 90 98 88436 92 88 96 85

The fused glass had the same optical transmission (92%) as theborosilicate glass in the above wavelength region of UV radiation. Theestimated transmission of the glass substrate with ITO on both sides iscalculated by squaring the transmission of the ITO (to account for thetwo layers) and then multiplying by the transmission of the substrate.The average transmission over the above spectral range is 85%. Theoptical transmission of the General Vacuum ITO and the Hoeschtphotoresist is 34% (0.4×0.85). Thus, the dose necessary to completelyexpose the bottom of the substrate would be at least about 3 times thedose used to expose the resist on one surface of the substrate for thesematerials.

Another photoresist may be obtained from Shipley (Microposit S1800) thathas an optical transmission after exposure of about 60% in the regionranging from 365 to 436 nm. The estimated transmission of the Shipleyphotoresist and the General Vacuum ITO is 51%. Therefore, the dosenecessary to completely expose the bottom of the substrate would be atleast about 2 times the dose used to expose the resist on one surface ofthe substrate.

The substrate side which was first coated with photoresist must beclosest to the mask during exposure, since a photosensitive compound inthe photoresist degrades with increased baking time. Exposure isincomplete if the side baked longer is placed on the bottom. The firstcoated photoresist which has been baked twice, by being on the top, seesa larger dose of radiation. This ensures that the now upper photoresistlayer is completely exposed despite its decreased sensitivity to the UVradiation. The UV dose necessary to completely expose the photoresist onthe top and bottom of the substrate is thus generally at least about 2times the dose normally used to expose the photoresist on one surface ofthe substrate.

After the exposure step, the photoresist pattern is developed, baked andthen placed in an acid bath to etch away unwanted regions of the ITO andcreate the electrode pattern on both sides of the substrate. The areawhere the radiation contacts the photoresist is washed away with adeveloper, one suitable developer being potassium hydroxide. Theelectrode under this region is removed with acid. One suitable acid is ahydrochloric/nitric acid solution. One part of an aqueous hydrochloricacid solution at a concentration of 37% by volume was added to one partof water. Added to this was 5% of an aqueous nitric acid solution at aconcentration of 70% by volume. The substrate is then cleaned and coatedwith a passivation layer 58 on one side and baked in the oven to cure.

The elongated electrode strips each had a width of about 244 microns. Aspace of about 15 microns between adjacent electrode strips was on oneside of the substrate and a space of about 20 microns between adjacentelectrode strips was on the other side. Of course, the spacing may bereduced by employing a collimated light source. The electrode stripshave a thickness of about 250 to about 275 microns. A resolution ofabout 100 dots per inch was obtained. Those skilled in the art wouldappreciate that this process may be modified to obtain displays havingdifferent electrode thickness, spacing and resolution.

The passivation layer is then coated on the opposite side and baked inan oven to cure. The alignment layer 60 is coated onto the substrate andcured in an oven. The alignment layer is then coated onto the oppositeside and baked in an oven to cure on shims so as not to damage theprevious alignment coating. Gasket seals are printed and spacers (notshown) are applied onto the outer substrates which have a patternedelectrode, passivation and alignment layers. The substrates are stackedtogether, aligned, pressure is applied to set the cell gap and thegasket seal is cured. The substrates are then cut to size. The liquidcrystal mixture is injected into each cell using a vacuum fillingtechnique. The cells are plugged, ledges are cleaned and black paint isapplied to the bottom substrate. The display is then bonded to the driveelectronics with a flexible connector.

The above photolithography technique produces the same electrode patternon both sides of a substrate. The display design must be compatible withthis approach. The existing design of a ⅛ video graphics adaptor (VGA)display was extended to use the three substrate stacked display byreplacing the bottom substrate, which would normally have the columnelectrodes, with a substrate with column electrodes and coatings on bothsides. The common substrate is sandwiched between identical substrateswith row electrodes, passivation and alignment layer coatings.

Referring to FIG. 10, the three substrates 46, 48 and 50 support spacedapart electrode matrices (not shown). Facing electrodes are alignedperpendicularly to each other and are individually energized by twodriver circuits 66, 68. The driver circuits are connected by edgecontacts 70, 72 to a logic circuit for controlling energization of thematrix arrays. Each of the driver circuits utilizes CMOS LCD driverchips to produce appropriate energization waveforms. Model SED 1744drivers for the column drivers and SED 1743 for the row drivers arecommercially available from S-MOS Systems, Inc., San Jose Calif. Datasheets for these circuits are incorporated herein by reference.

The stacked display may also be fabricated to reflect multiple colors.In this regard, two, three or more cells that reflect visible light maybe used. FIG. 11 shows one example of a stacked multi-color display.First, second and third visible reflecting cells 80, 82 and 84 arestacked in series in front of an infrared reflecting cell 86. Thedisplay includes substrates 88, 90, 92, 94 and 96. Substrate 88 isdisposed closest to an observer at the front of the cell and thesubstrate 96 is disposed at the back of the display. First, second andthird chiral nematic liquid crystal materials 100, 102 and 104 have apitch length effective to reflect visible light. Liquid crystal material106 has a pitch length effective to reflect infrared radiation.

This particular display employs substrates having electrodes on bothsides, prepared according to the photolithography method of the presentinvention. However, the arrangement shown in FIG. 5 may be employed aswell, in which case eight substrates may be used. Index matchingmaterial would then be employed between adjacent substrates. Passivationand alignment layers are also disposed on the substrates.

Each of the liquid crystals 100, 102 and 104 has a concentration ofchiral additive that produces a pitch length effective to reflect adifferent wavelength of visible light than the others. The liquidcrystal compositions may be designed to reflect light of any wavelength.For example, the first cell 80 may reflect red light, the second cell 82may reflect blue light and the third cell 84 may reflect green light. Inaddition, to achieve a brighter stacked cell display, the liquid crystalin one cell may have a different twist sense than the liquid crystal ofan adjacent cell for infrared/visible displays and color displays. Forexample, in a three cell stacked display, the top and bottom cells mayhave a right handed twist sense and the middle cell may have a lefthanded twist sense.

The back substrate of each cell may be painted a particular color or aseparate color imparting layer 108 may be used. Examples of colorimparting layers suitable for use in the present invention are providedin U.S. Pat. No. 5,493,430, entitled “Color, Reflective Liquid CrystalDisplays,” which is incorporated herein by reference in its entirety.The back substrate of the visible cell that is furthest from theobserver may be painted black or a separate black layer may be used toimprove contrast, replacing layer 108.

The bistable chiral nematic liquid crystal material may have either orboth of the focal conic and twisted planar textures present in the cellin the absence of an electric field. In a pixel that is in thereflective planar state, incident light is reflected by the liquidcrystal at a color determined by the selected pitch length of that cell.If a color layer or “backplate” 108 is disposed at the back of thatcell, light that is reflected by the pixel of that cell in thereflective planar state will be additive of the color of the liquidcrystal and the color of the backplate. For example, a blue reflectingliquid crystal having an orange backplate will result in a generallywhite light reflected from the pixel in the reflective planar state. Apixel of the cell that is in the generally transparent focal conic statewill reflect the orange color of the backplate to produce a white onorange, orange on white display. If a black layer is used at the back ofthe cell, rather than a colored backplate, the only color reflected willbe that of the planar texture of the liquid crystal, since the blacklayer absorbs much of the other light. The color imparting layers of thevisible cells and the black layer at the back substrate of the lastvisible cell are transparent so to enable light to travel to the nextcell.

In the case of two or more cells, some incident light is reflected bythe planar texture of the first cell at a particular color. Two or eventhree of the cells may be electrically addressed so as to have theirliquid crystal transformed into the reflecting planar state, in whichcase the color reflected from the display would be produced by additivecolor mixing. Since not all of the incident light is reflected by theliquid crystal of the first cell, some light travels to the second cellwhere it is reflected by the planar texture of the second cell. Lightthat travels through the second cell is reflected by the planar textureof the third cell at a particular color. The color reflected by thefirst, second and third cells is additively mixed. The invention canreflect the colors of selected cells by only transforming the particularcell into the reflecting planar texture, the other cells being in thefocal conic state. In this case, the resultant color may be monochrome.

Moreover, by utilizing grey scale by a process such as that disclosed inthe U.S. Pat. No. 5,453,863, one or more cells of the display may bemade to reflect light having any wavelength at various intensities.Thus, a full color display may be produced. The display may also be madeto operate based upon principles of subtractive color mixing using abacklighting mode. The final color that is produced by variouscombinations of colors from each liquid crystal material, differentcolored backplates, and the use of grey scale, can be empiricallydetermined through observation. The entire cell may be addressed, or thecell may be patterned with electrodes to form an array of pixels, aswould be appreciated by those skilled in the art in view of thisdisclosure. The driver electronics for this display would be apparent tothose skilled in the art in view of this disclosure.

The spacing between substrates of the visible cells of FIG. 11 isuniform. However, the visible cell spacing may be adjusted as desired.For example, a cell that reflects blue light employs a relatively smallpitch length. Therefore, the cell spacing needed to accommodate enoughpitches for suitable reflectance may be decreased. As a result, the cellmay have a smaller spacing, which enables the cell to be driven at alower voltage than the cells having a larger spacing.

Two, three or more visible cells may be employed in conjunction with theinfrared cell, as shown in FIG. 11. Alternatively, a display may includetwo, three or more visible cells without an infrared cell. The design ofsuch a display may be similar to that shown in FIG. 5, except that theinfrared cell would be replaced by a cell that reflects visible light.The liquid crystal composition, composition of additives, cellfabrication and operation of such a stacked multiple color, visible celldisplay would be apparent to those skilled in the art in view of thisdisclosure.

The chiral nematic liquid crystal compositions suitable for use in thepresent invention may vary depending upon their use in the single cellor the stacked cell display. In the case of the single cell display theliquid crystal composition generally comprises a chiral nematic materialranging from about 58 to about 70%. Nematic material may be used in therange of from about 30 to about 42%. All amounts of materials providedherein are in % by weight unless otherwise indicated.

In the case of the stacked cell display, each visible cell comprises aliquid crystal material generally comprising chiral nematic materialranging from about 70 to about 100% and nematic material ranging from 0to about 30%. The infrared cell has a liquid crystal compositioncomprising chiral nematic material ranging from not greater than about58% and a nematic material ranging from at least about 42%. The nematicmaterial may be added to adjust the concentration of the chiral materialand thus, the pitch length of the composition. Alternatively, it will beappreciated that a chiral dopant may be added to a base nematic materialin specific amounts to produce the desired pitch length.

The bistability of the liquid crystal composition may be obtained usinga polymer network or surface treatment, but requires neither. Thepolymer stabilized cholesteric texture (PSCT) displays employ substrateshaving surface treatment that promotes homogeneous alignment, with theliquid crystal material including small amounts of monomer andphotoinitiator. For a description of suitable polymer stabilizedcompositions and their cell fabrication, refer to the Doane and Yangarticles cited above as well as to U.S. Pat. No. 5,570,216, entitled“Bistable Cholesteric Liquid Crystal Displays with Very High Contrastand Excellent Mechanical Stability;” U.S. Pat. No. 5,251,048, entitled“Method and Apparatus for Electronic Switching of a ReflectiveCholesteric Display” and U.S. Pat. No. 5,384,067, entitled “Grey ScaleLiquid Crystal Material,” which are incorporated herein by reference intheir entireties.

As an example of suitable components for a polymer stabilizedcomposition, the monomer may be used in amounts ranging from about 1.0to about 1.2% by weight based upon the total weight of the composition,one example being 4,4′-bisacryloylbiphenyl, synthesized by Kent StateUniversity. The photoinitiator is used in an amount ranging from about0.25% to about 0.3% by weight based on the total weight of thecomposition, suitable examples being IRGACURE® 369 and 651 brandphotoinitiators obtained from Ceiba-Geigy Corp. The amounts of chiralnematic material and nematic material in the PSCT composition may bedecreased by about 1.5% from the standard composition.

Regarding the polymer free compositions, in some instances it isdesirable to treat the cell walls and the electrodes with materials,such as the passsivation and alignment layers referred to above.Detergents or chemicals may be used to treat the cell walls to obtainvariations in the contrast or switching characteristics. Thesetreatments can be used to affect the uniformity of the liquid crystal,alter the stability of the various textures and to alter the strength ofany surface anchoring. In addition to using a wide variety of materialsfor such surface treatments, the treatments on opposing substrates maydiffer. For example, the substrates may be rubbed in differentdirections, one substrate may include the treatment while the other maynot, or opposite substrates may be used with different materials. Asnoted above, such treatments can have the effect of altering the effectof the cell response. The passivation layer or electrode material alonemay sufficiently stabilize the focal conic texture. Optionally, otheradditives may be included in the chiral nematic liquid crystal mixtureto alter the characteristics of the cell. For example, while color isintroduced by the liquid crystal material itself, pleochroic dyes may beadded to intensify or vary the color reflected by the cell. Similarly,additives such as fumed silica can be dissolved in the liquid crystalmixture to adjust the stability of the various cholesteric textures.

The present invention will now be described by reference to thefollowing nonlimiting examples.

EXAMPLE 1

A stacked display was fabricated using the following compositions, in %by weight based on the total weight of the composition:

Visible Cell BL100 75.75% BL101 24.00% G-232 0.250% Infrared Cell BL061 48.0% E44  52.0%

In the above compositions, all liquid crystal materials were obtainedfrom Merck, Ltd. United Kingdom. BL061 and BL100 are chiral nematicmaterials and BL101 and E44 are nematic materials. G-232 is a dychroicdye obtained from Nippon Kankoh-Shikiso Kenkyusho Co., Ltd., Japan. Foursubstrates were used. Each substrate had an ITO coating, a passivationlayer and an alignment layer, respectively, in a direction away from thesubstrate. The alignment layer stabilizes the focal conic texture andprovides the liquid crystal adjacent the layer with generallyhomeotropic alignment. The alignment layer was SE-7511L from NissanChemical. The cells were obtained from Varitronix Ltd. and CrystalloidElectronics. The infrared cell also had black paint applied to its outersurface. The liquid crystal that reflects visible light was filledthrough a port between two opposing substrates and the liquid crystalthat reflects infrared radiation was filled through a port between twoopposing substrates using a vacuum filling technique. The visible cellwas filled in about an hour and the infrared cell, having a lowerviscosity, was filled in less time. The visible cell had a pitch lengthof about 0.36 microns and the infrared cell had a pitch length of about0.56 microns. The ports of both cells were sealed. An index matchingglycerol fluid was disposed between the cells, forming a stackeddisplay. A two part epoxy adhesive was used on the perimeter of thecells.

EXAMPLE 2

A composition for a single cell display that reflects both visible andinfrared radiation included, in % by weight based on the total weight ofthe composition: BL061 chiral nematic material in an amount of about60%, E44 nematic material in an amount of about 40%. A dye may also beused for improving the contrast of the reflected visible radiation. Forexample, a blue absorbing dye may be used, the dye eliminating thescattering of blue light at a wavelength of about 500 nm. Thiscomposition was tailored to have an optical anisotropy of about 0.26.The manufacturer, Merck, Ltd. is able to design the chiral nematiccomposition to have the desired optical anisotropy.

Grey Scale

The following text is taken from the U.S. Pat. No. 5,453,863, toillustrate how the stacked color display of the invention may utilizegrey scale. By utilizing grey scale by a process such as that disclosedin the U.S. Pat. No. 5,453,863, one or more cells of the display may bemade to reflect light having any wavelength at various intensities.

As disclosed in the U.S. Pat. No. 5,453,863, a reflective color displaycell can be prepared without polymer so that it exhibits multipleoptically different states, all of which are stable in the absence of anapplied field. The display can be driven from one state to another by anelectric field. Depending upon the magnitude and shape of the electricfield pulse, the optical state of the material can be changed to a newstable state which reflects any desired intensity of colored light alonga continuum of such states, thus providing a stable “grey scale.”Surprisingly, these materials can be prepared without the need forpolymers and the added expense and manufacturing complexities associatedtherewith.

Generally, a sufficiently low electric field pulse applied to thematerial results in a light scattering state which is white inappearance. In this state, a proportion of the liquid crystal moleculeshave a focal conic texture as a result of competition between anysurface effects, elastic forces and the electric field. Afterapplication of a sufficiently high electric field pulse, i.e., anelectric field high enough to homoeotropically align the liquid crystaldirectors, the material relaxes to a light reflecting state that can bemade to appear as green, red, blue, or any pre-selected color dependingupon the pitch length of the chiral nematic liquid crystal. The lightscattering and light reflecting states remain stable at zero field. Bysubjecting the material to an electric field in between that which willswitch it from the reflecting state to the scattering state, or viseversa, one obtains stable grey scale states characterized by varyingdegrees of reflection in between that exhibited by the reflecting andscattering states. When the chiral nematic liquid crystal is in a planarcolored light reflecting texture and an intermediate electric fieldpulse is applied, the amount of material in the planar texture and theintensity of reflectivity of the colored light, decrease. Similarly,when the material is in the focal conic texture and an intermediateelectric field pulse is applied, the amount of material in the planartexture will increase as will the intensity of reflection from the cell.When the electric field is removed, the material is stable and remainsin the established texture to reflect that intensity of lightindefinitely, regardless of which texture it started from.

If an electric field high enough to homeotropically align the liquidcrystal directors is maintained, the material is transparent until thefield is removed. When the field is turned off quickly, the materialreforms to the light reflecting state and, when the field is turned ofslowly, the material reforms to the light scattering state. In eachcase, the electric field pulse is preferably an AC pulse, and morepreferably a square AC pulse, since a DC pulse will tend to cause ionicconduction and limit the life of the cell.

While not wanting to be bound by theory, it is believed that when thevoltage is applied, a proportion of the material enters a turbid phasewhile the field is on. Those portions of the material that exhibit theturbid phase tend to relax to a focal conic, light scattering textureupon removal of the field. Those portions of the material unaffected bythe field, i.e., those portions that do not enter the turbid phase,remain in the planar, light reflecting texture. The amount of lightreflected from the cell depends on the amount of material in the planarreflecting texture. When the voltage of the electric field is increased,a higher proportion of the material enters the turbid phase while thefield is on, followed by relaxation to the focal conic texture when thefield is removed. Since the reflection from the cell is proportional tothe amount of material in the planar reflecting texture, reflection fromthe cell decreases along a grey scale as a result of an increase in themagnitude of the field because more of the material enters the turbidphase and is switched to the focal conic texture. At a certain thresholdvoltage, which depends upon the material, substantially all of thematerial is switched to the focal conic texture upon removal of thefield, characterized by a light scattering condition where thereflectivity of the cell is at or near a minimum. When the voltage isremoved, the assumed texture is stable and will remain scatteringindefinitely. When the voltage is increased further, to a point highenough to untwist the liquid crystal and homeotropically align theliquid crystal directors, the material is transparent and will remaintransparent until the voltage is removed. From the homeotropic texture,the material tends to relax to the stable color reflecting planartexture upon removal of the field.

When the material is in a light scattering focal conic texture and a lowvoltage pulse is applied, the material begins to change texture andagain stable grey scale reflectivities are obtained. Since the materialhere starts in the scattering focal conic texture, the grey scalereflectivities are characterized by an increase in the reflectivity fromthat exhibited when substantially all of the material is in thescattering focal conic texture, although it has been observed that thereflectivity may initially decrease in some samples. The increase inreflectivity is believed to be attributable to proportions of thematerial that become homeotropically aligned as a result of the appliedfield. Those proportions that are homeotropically aligned relax to astable planar light reflecting texture upon removal of the field, whilethe remainder of the material exhibits the turbid phase as a result ofthe field and relaxes back to the focal conic texture upon removalthereof. When the voltage is increased still further to the point ofhomeotropically aligning substantially all of the liquid crystal, thematerial again appears clear and relaxes to the stable planar colorreflecting texture upon removal of the field.

In short, it is believed that those proportions of the material thatenter the turbid phase as the result of an applied field relax to astable focal conic texture upon removal of the field and those portionsthat become homeotropically aligned due to the application of an appliedfield relax to a stable planar texture upon removal of the field. It isbelieved that the material returns to the scattering focal conic statewhen a high electric field is slowly removed from the homeotropicallyaligned liquid crystal because slow removal takes the material into theturbid phase from which it seems to consistently relax to a focal conictexture after removal of a field. When a high field is removed quickly,the material does not enter the turbid phase and thus, relaxes to theplanar reflecting texture. In any case, it can be seen that electricfield pulses of various magnitudes below that necessary to drive thematerial from the stable reflecting state to the stable scatteringstate, or vise versa, will drive the material to intermediate statesthat are themselves stable. These multiple stable states indefinitelyreflect colored light of an intensity between that reflected by thereflecting and scattering states. Thus, depending upon the magnitude ofthe electric field pulse, the material exhibits stable grey scalereflectivity without the need for polymer. The magnitude of the fieldnecessary to drive the material between various states will, of course,vary depending upon the nature and amount of the particular liquidcrystal and thickness of the cell. Application of mechanical stress tothe material can also be used to drive the material from a lightscattering to a light reflecting state.

Example 1 of the U.S. Pat. No. 5,453,863

A chiral nematic liquid crystal mixture containing 37.5% by weight E48(nematic liquid crystal from EM Chemicals) and 62.5% by weight TM74A(chiral additive from EM Chemicals) was prepared. A one inch square cellwas then formed from two substrates coated with ITO. The ITO coatings ofboth substrates were buffed parallel to each other. 10 μm glass spacerswere sprayed onto one substrate and the second substrate was sandwichedso that two of its edges overlapped the first substrate and the cellheld together with clamps. Five minute epoxy (Devcon) was then used toseal the two nonoverlapping edges.

The cell was held vertically and a bead of the chiral nematic liquidcrystal was placed along the top open edge of the cell. The cell thenfilled spontaneously by capillary action over a period of approximately15 minutes. Once filled, the residual liquid crystal mixture is removedfrom the edge and the open edges sealed with five minute epoxy.

The cell was initially in the planar reflecting state. A 100 ms lowervoltage pulse of about 115 volts and 1 KHz. switched the cell into thefocal conic scattering state. A 100 ms higher voltage pulse of about 180volts and 1 KHz switched the cell back to the planar reflecting state.Both the planar and focal conic states were stable in the absence of afield and the cell exhibited multiple stable grey scale reflectingstates between the scattering and reflecting states.

Example 2 of the U.S. Pat. No. 5,453,863

A mixture of E48 and TM74A in a weight ratio of 0.6:1 was introducedbetween ITO coated glass substrates spaced 10 micrometers apart as inthe previous example. The substrates were additionally coated with anunrubbed polyamide layer. The cell was initially in the focal conic,scattering texture that transmitted only about 30% of an HeNe beamthrough the cell. A 10 ms. 155 volt 1 KHz Ac pulse switched the cell toa planar texture reflecting green colored light. The transmission fromthe cell in the reflecting state was about 65%. A 95 volt pulse of thesame duration and wavelength switched the cell back up the focal conic,scattering state. The cell switched between states in less than 10 ms.

Example 3 of the U.S. Pat. No. 5,453,863

A cell was prepared as in the preceding examples with a mixture of CB15,CE2 (chiral materials from EM Chemicals) and E48 nematic liquid crystalin a weight ratio of 0.15:1.15:0.7. In this cell the driving voltage wascut approximately in half because the dielectric anisotropy of themixture was higher then when TM74A was used. The electro-optic responseof this material was similar to that of Example 1.

Table I shows numerous additional examples of materials preparedaccording to the preceding examples. The concentration of chiralmaterial, and the type and concentration of nematic liquid crystal werevaried in these cells. In each case the chiral material was a 50:50mixture of CE2 and CB 15. Each cell employed unrubbed ITO electrodes asthe only surface treatment on the substrates. The materials in Table Iall exhibited multistability in the visible spectrum, i.e., stablereflecting, scattering and grey scale states.

TABLE I TABLE I of the 5,453,863 Patent Chiral Nematic Thick- Multi-Agent LC ness Color stability Surface CE2/C815 E48 70% 10 μm Red Yes ITO30% CE2/CB15 E48 60% 10 μm Grn Yes ITO 40% CE2/CB15 E48 50% 10 μm BluYes ITO 50% CE2/CB15 E7 70% 10 μm Red Yes ITO 30% CE2/CB15 E7 60% 10 μmGrn Yes ITO 40% CE2/CB15 E31 70% 10 μm Red Yes ITO 30% CE2/CB15 E31 60%10 μm Grn Yes ITO 40%

The polymer free multistable color display cells of the inventionexhibit a stable grey scale phenomenon characterized by the ability ofthe material to reflect indefinitely any selected intensity of lightbetween the intensity reflected by the reflecting state and thatreflected by the scattering state, the former being when substantiallyall of the material exhibits the planar texture and the latter beingwhen substantially all of the material exhibits the focal conic texture.For purposes of this invention, the reflecting state reflects coloredlight at a maximum intensity for a given material, the color of thereflected light being determined by the pitch length of the chiralmaterial. An electric field pulse of an appropriate threshold voltagewill cause at least a portion of the material to change its opticalstate and the intensity of reflectivity to decrease. If the AC pulse ishigh enough, but still below that which will homeotropically align theliquid crystal, the optical state of the material will change completelyto the scattering state which reflects light at a minimum intensity fora given material. In between the reflecting state, which for a givenmaterial can be considered to define the maximum intensity ofreflectivity for that material, and the scattering state, which can beconsidered to define the minimum intensity of reflectivity, theintensity of reflectivity ranges along a grey scale, which is simply acontinuum of intensity values between that exhibited by the reflectingand scattering states. By pulsing the material with an AC pulse of avoltage in between that which will convert the material from thereflecting state to the scattering state, or visa versa, one obtains anintensity of reflectivity in this grey scale range.

While not wanting to be bound by theory, it has been observed that theintensity of reflectivity along the grey scale when the material beginsin the planar texture is approximately linearly proportional to thevoltage of the pulse. By varying the voltage of the pulse the intensityof reflectivity of a given color can be varied proportionally. When theelectric field is removed the material will reflect that intensityindefinitely. It is believed that pulses within this grey scale voltagerange cause a proportion of the material to convert from the planartexture characteristic of the reflecting state, to the focal conictexture characteristic of the scattering state. The intensity ofreflectivity along the grey scale is proportional to the amount ofchiral material switched from the planar texture to the focal conictexture or vice versa, which is in turn proportional to the voltage ofthe AC pulse.

In the light reflecting state, the chiral liquid crystal molecules 40are oriented in a twisted planar structure parallel to the cell walls.Because of the twisted planar texture the material will reflect light,the color of which depends upon the particular pitch length. In thisstable reflecting state, the material exhibits maximum reflectivity thatconstitutes a maximum reference intensity below which the grey scaleintensities are observed. The planar texture of the liquid crystal isstable without the presence of polymer. In the stable light scatteringstate the material exhibits its minimum intensity of reflection (i.e.,maximum scattering) which defines a minimum reference intensity ofreflectivity above which the grey scale intensities are observed.

Both the light reflecting state and the light scattering state as wellas the grey scale states therebetween, are stable in the absence of anelectric field. If the material is in the light reflecting state and alow electric field pulse is applied, the material will be driven to thelight scattering state and will remain in that state at zero field. Ifthe multistable material is in the light scattering state and a higherelectric field pulse sufficient to untwist the chiral molecules isapplied, the liquid crystal molecules will reform to the lightreflecting state at the end of the pulse and will remain in thatcondition. It is to be understood that the voltages per micron of cellthickness necessary to drive the material between optical states mayvary depending on the composition of the material, but that thedetermination of necessary voltages is well within the skill in the artin view of the instant disclosure.

If the high electric field necessary to untwist the liquid crystalmolecules is maintained, the liquid crystal directors will behomeotropically aligned so that the material is transparent. If thefield is slowly removed, the liquid crystal orientation will reform tothe light scattering state presumably because slow removal allows asignificant proportion of the material to enter the turbid phase. Whenthe field is quickly removed, the orientation will reform to the lightreflecting state. The intensities of reflectivity reflected between thereflecting state and the scattering state are stable grey scalereflectivities. Of course, the intensity value of the reflecting andscattering states may vary as the composition of the material varies,but the grey scale is defined by the range of intensities therebetween.

At voltages less than that which will transform the material from thereflecting state to the scattering state, grey scale states which arethemselves stable at zero field are obtained. The reflection from thematerial in these grey scale states is stable because a proportion ofthe material is in the planar reflecting texture and a proportion of thematerial is in the focal conic scattering texture both of which arestable in the absence of a field.

Thus, for example, if the material is in the reflecting state and anelectric field pulse is applied having a voltage insufficient to driveall of the liquid crystal into the focal conic texture, i.e.,insufficient to drive the material completely to the scattering state,the material will reflect colored light of an intensity that isproportional to the amount of material that remains in the planarreflecting texture. The reflectivity will thus be lower than thatreflected from the material when all of the chiral nematic liquidcrystal is in the planar reflecting texture, but still higher than whenswitched completely to the focal conic scattering texture. As thevoltage of the electric field pulse is increased, more of the chiralmaterial is switched from the planar reflecting texture to thescattering focal conic texture and the reflectivity decreases furtheruntil the voltage of the pulse is increased to the point where all ormost of the material enters the turbid phase from which it relaxes andis completely switched to the scattering state. If the voltage of thepulse is increased still further, the intensity of reflection begins toincrease again until the magnitude of the pulse is sufficient to untwistmost of the chiral molecules so that they will again reform to theplanar light reflecting texture when the pulse is quickly removed andthe material is again in the light reflecting state.

If the material is in the focal conic scattering state, an appliedelectric field pulse will have a much less dramatic effect on thereflectivity of the cell than when it starts in the planar texture,until the voltage reaches a magnitude sufficient to untwist the chiralmaterial, whereby it will reform to the light reflecting state asdescribed above, when the field is removed. Grey scale when the materialstarts in the focal conic texture appears to result when a proportion ofthe molecules untwist and homeotropically align as a result of theapplication of the field. This proportion of molecules then relaxes tothe planar reflecting texture upon removal of the field.

The response of a cell as described above is illustrated in FIG. 12which shows the response of the material prepared in Example 1 of theU.S. Pat. No. 5,453,863 to varying pulse voltages.

The reflectivity of the cell in response to AC pulse of varying voltageswas measured. In the measurement, 100 millisecond, 1 KHz AC pulses wereused. For this material an applied pulse above about 180V switched thecell into the reflecting state independent of whether the cell was inthe scattering or reflecting state prior to the pulse. Maximumreflection, i.e., transmission, is observed here. The material exhibitedmaximum scattering when a voltage in the 130 to 140V range was applied,regardless of whether the material was in the planar or focal conictexture prior to the pulse.

The grey scale response of the cell in response to pulses of varyingvoltage is also seen in FIG. 12. Here the voltage of the pulse wasvaried and the reflection (% transmission) from the cell was measured.Curve A is the response of the cell when the material is in thereflecting state prior to each pulse. Prior to each pulse plotted oncurve A the material was subjected to a high AC pulse to ensure that itwas completely in the reflecting state prior to the pulse. When thevoltage of the pulse is below about 30V, the reflection of the cell isnot significantly affected. When the voltage of the pulse is betweenabout 40V and 110V, the reflectivity of the cell decreases approximatelylinearly as the voltage of the pulse is increased. Grey scalereflectivity is observed in this voltage range. In each case thematerial continued to reflect after the pulse was removed. When thevoltage of the pulse was increased to from about 120 to 130V, thematerial was in the scattering state and exhibited near maximumscattering. When the magnitude of the pulse was increased still furtherabove about 150 to 160V, the reflectivity of the cell increased untilthe reflectivity approximated its original value, i.e., that of thereflecting state, above 180V.

Curve B shows the response of the cell when the material was initiallyin the focal conic scattering state prior to the AC pulse. Here thereflectivity of the cell does not significantly change for AC pulsesbelow about 30V. Between about 50 and 150V the scattering actuallyincreases slightly and maximum scattering is observed from the cell.Above about 160V the transmission quickly increased and the cellswitched to the reflecting state approximating the maximum transmissionabove about 180V.

It can be seen that the linear relationship of the grey scale to voltageis much more pronounced, and the grey scale more gradual, when thematerial starts from the planar texture. Accordingly, most practicalapplications of the grey scale phenomenon will likely employ thematerial starting from the planar texture.

Many modifications and variations of the invention will be apparent tothose of ordinary skill in the art in light of the foregoing disclosure.Therefore, it is to be understood that, within the scope of the appendedclaims, the invention can be practiced otherwise than has beenspecifically shown and described.

1. A method of producing additive and monochrome color images from astacked chiral nematic liquid crystal display, comprising: passingincident light into a first layer of the display, said first layercomprising first chiral nematic liquid crystal material, the liquidcrystal of said first material having a pitch length effective toreflect visible light of a first color; electrically addressing saidfirst layer to reflect visible light from the display of said firstcolor by transforming a portion of the liquid crystal of said firstlayer into a reflective twisted planar texture; passing said incidentlight through said first layer into a second layer of the displaystacked relative to said first layer, said second layer comprisingsecond chiral nematic liquid crystal material, the liquid crystal ofsaid second material having a pitch length effective to reflect visiblelight of a second color; electrically addressing said first layer andsaid second layer to reflect visible light from the display of saidsecond color by transforming a portion of the liquid crystal of saidsecond layer into the reflective twisted planar texture and a portion ofthe liquid crystal of said first layer into the focal conic texture; andelectrically addressing said first layer and said second layer toreflect visible light from the display of a color that is additive ofsaid first color and said second color by transforming portions of theliquid crystal of said first layer and said second layer into thereflective twisted planar texture.
 2. The method of claim 1 comprising:passing said incident light through said first layer, through saidsecond layer and into a third layer of the display stacked relative tosaid first layer and said second layer, said third layer comprisingthird chiral nematic liquid crystal material, the liquid crystal of saidthird material having a pitch length effective to reflect visible lightof a third color; electrically addressing said first layer, said secondlayer and said third layer to reflect visible light from the display ofsaid third color by transforming a portion of the liquid crystal of saidthird layer into the reflective twisted planar texture and a portion ofthe liquid crystal of said first layer and said second layer into thefocal conic texture; electrically addressing said first layer, saidsecond layer and said third layer to reflect visible light from thedisplay of a color that is additive of said third color and said secondcolor by transforming portions of the liquid crystal of said third layerand said second layer into the reflective twisted planar texture and aportion of the liquid crystal of said first layer into the focal conictexture; electrically addressing said first layer, said second layer andsaid third layer to reflect visible light from the display of a colorthat is additive of said third color and said first color bytransforming portions of the liquid crystal of said third layer and saidfirst layer into the reflective twisted planar texture and a portion ofthe liquid crystal of said second layer into the focal conic texture;and electrically addressing said first layer, said second layer and saidthird layer to reflect visible light from the display of a color that isadditive of said first color, said second color and said third color bytransforming portions of the liquid crystal of said first layer, saidsecond layer and said third layer into the reflective twisted planartexture.
 3. The method of claim 1, wherein said incident light travelsin a direction sequentially through said first region and said secondregion, said first region being closest to a viewer, comprising a lightabsorbing layer disposed downstream of said second region relative tosaid direction of incident light; and absorbing said incident lightpassing through said first region and said second region with said lightabsorbing layer.
 4. The method of claim 2 wherein incident light travelsin a direction sequentially through said first region, said secondregion and said third region, said first region being closest to aviewer, comprising a light absorbing layer disposed downstream of saidthird region relative to said direction of incident light; and absorbingsaid incident light passing through said first region, said secondregion and said third region with said light absorbing layer.
 5. Themethod of claim 1 wherein said first layer and said second layer have auniform thickness of said first liquid crystal material and said secondliquid crystal material, respectively, across an entire viewing area ofthe display.
 6. The method of claim 2 wherein said first layer, saidsecond layer and said third layer have a uniform thickness of said firstliquid crystal material, said second liquid crystal material, and saidthird liquid crystal material, respectively, across an entire viewingarea of the display.